This invention relates to an improved method of papermaking in an acid or alkaline system using a selected crosslinked, cationic or amphoteric, jet cooked starch as a wet end additive.
The term "paper," as used herein, includes sheet-like masses and molded products made from fibrous cellulosic material, which may be derived from natural sources, synthetics such as polyamides, polyesters, rayon and polyacrylic resins as well as from mineral fibers such as asbestos and glass. In addition, paper made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term "paper".
Papermaking, as it is conventionally known, is a process of introducing an aqueous slurry of pulp or wood cellulosic fibers (which have been beaten or refined to achieve a level of fiber hydration and to which a variety of functional additives can be added) onto a screen or similar device in such a manner that the water is removed, thereby forming a sheet of the consolidated fibers, which upon pressing and drying can be processed into dry roll or sheet form. Two well known papermaking operations involve the Fourdrinier machine, the most common, and the cylinder machine. In the Fourdrinier and multicylinder operations, and in other machine operations, as typical in papermaking, the feed or inlet to the machine is an aqueous slurry or water suspension of pulp fibers which is provided from what is called the "wet end" system. In the wet end, the pulp along with other additives are mixed in an aqueous slurry and subject to mechanical and other operations such as beating and refining to improve interfiber bonding and other physical properties of the finished sheet. Additives commonly introduced along with the pulp fibers are pigments such as titanium dioxide, mineral fillers such as clay and calcium carbonate and other materials introduced into paper to achieve such properties as improved brightness, opacity, smoothness, ink receptivity, fire retardance, water resistance, increased bulk, etc.
Starch has been used in the paper industry for many years and in fact, is the second largest volume raw material component in paper. Starches perform two of the most important items required in papermaking; strength improvement and increased drainage on the wire. Both unmodified and modified types have been used. However, due to the complexity of today's pulp furnishes, the sizing chemicals and other chemicals present, cationic modified starches are preferred since they are retained to a high degree by the paper machine furnish.
Various cationic starches are known and used in the paper industry with the tertiary amino and quaternary ammonium starch ethers being the most commercially significant derivatives. These and other cationic starches as well as the method of preparing them are described in "Cationic Starches" by D. B. Solarek, Modified Starches: Properties and Uses, Chapter 8, pp. 113-129, 1986.
Crosslinking is known to affect both the textural and viscosity properties of starch. It strengthens and helps hold the starch granules together. While the crosslinking of starch, either by itself or in combination with other starch modifications, has been used in a number of applications, it has not been used to a large extent in papermaking. An early patent, U.S. Pat. No. 3,417,078 issued Dec. 17, 1968 to C. Patel discloses the use of a cationic starch in the manufacture of paper, the starch being a selected imidazoline derivative which is also reacted with a crosslinking agent such as dichlorobutene. Another patent publication, EPO 097,371 published Jan. 4, 1984 to S. Frey discloses the use of nongelatinized starch which is cationized and partly crosslinked in a papermaking process.
Two recent patents further disclose the use of cationized/crosslinked starches in papermaking processes. U.S. Pat. No. 5,122,231 issued Jun. 16, 1992 to K. Anderson discloses an improved method of papermaking wherein a cationic starch is subsequently crosslinked after cationization and then added to the wet end of the papermaking system to provide increased starch loading capacity. Japanese Patent Disclosure No. 2-133695, published May 22, 1990 to K. Maeda discloses a method of making paper wherein a cationic crosslinked starch having specified but broad degrees of crosslinking and cation substitution are used.
While the general applicability of using crosslinked, cationized starch in papermaking has been disclosed previously as noted above, the capability of providing significant process improvement was not demonstrated.